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New Zealands 5 worst weeds
Aug 15, 2003

Water hyacinth, cape tulip, water lettuce, salvinia and Johnson grass are officially NZ’s worst weeds. These five are all classed as notifiable organisms – this means that you must report ANY occurrences of these plants to MAF or the local biosecurity officer, who will promptly arrange to eradicate the infestation.

Prior to legislation change, notifiable organisms were referred to as “Class A noxious plants”. Notifiable organisms (see Section 45 of the Biosecurity Act) are organisms that; are not established in New Zealand, are of extremely limited distribution, or are now eradicated from the country. Any reported sightings are immediately investigated and controlled by the Government. Notifiable organisms are all species that have been identified as being an intolerable threat to NZ and which are capable or potentially capable of causing unwanted harm to animal or plant production or market access. All notifiable weeds are first declared as unwanted organisms (those plants that are banned from sale, distribution and propagation). They become notifiable through an Order in Council. In simple terms, notifiable weeds are plants that the government will investigate and control immediately if they are found in the country.

So, how did such nice looking plants become infamous Notifiable Organisms? Essentially their status is determined through observations of their behaviour – both overseas and here in New Zealand.

For example; water hyacinth (Eichhornia crassipes) is an aquatic weed in at least 56 countries where it has demonstrated an incredible ability to spread. In Florida, just 70 years after water hyacinth was first introduced it occupied about 51,000 ha. This, together with the discovery that its seeds are extremely long lived, helped formulate the decision to ban it from sale, distribution and propagation in New Zealand. Unfortunately, being an attractive plant, it had already been introduced for ornamental plantings and had spread to several sites in the North Island. It is now mostly eradicated from these sites.

Salvinia (Salvinia molesta), with its dense floating mats and ability to double in volume in 2-3 days, is known as one of the world’s most damaging aquatic weeds. In Australia it is a Weed of National Significance due to its ability to smother large water bodies affecting water quality, water flow, wildlife, irrigation, and recreational activities. In New Zealand it has been found in fish ponds as far south as Wellington, and has been reported growing wild from the Bay of Plenty north. Most of these sites have been clear of the plant for at least 50 years and are referred to as “historical”.

Of the five notifiable weeds, only water lettuce (Pistia stratiotes) is thought to have been successfully eradicated from the country. Water lettuce (named due to its lettuce-like appearance) is another aquatic weed that seriously affects water quality, wildlife, irrigation and recreational activities. In tropical parts of Australia, water lettuce can actually cover large sections of rivers, dams and canals. In New Zealand, water lettuce is only likely to establish in northern parts of the north island.

Johnson grass and Cape tulip are certainly not limited to the North Island, and Canterbury has the dubious honour of being the place where these species have been most commonly found. Cape tulip (Homeria collina), which can cause gastroenteritis, thirst, paralysis, blindness and heart /kidney failure, often occurs in urban areas and has been found in both the South and North Islands. Cape tulip is a member of the iris family and like other irises dies back to an underground corm (in early summer). In one hectare of Cape tulip infested land, up to 25 million corms can be found. These corms remain dormant in the soil for at least 8 years, making control and eradication difficult – and giving Cape tulip the potential to have a huge impact on agricultur